Decarb and Compression

[fishchaser]

History: 1955 QD16 10hp Johnson. New impeller, plugs, points, condensor, coil and rebuilt carb. Running pretty good. Could see carbon deposits on tops of pistons thru plug hole. Approx 60psi compression on top and bottom cylinder.

Performed Deep Creep decarb today. Sprayed heavy doses into carb for about 2 minutes. Then sprayed enough to stall engine. Pulled plugs. Sprayed large amounts into each cylinder. Let sit one hour. Fired it up. Sprayed some more in carb. LOTS of blue/black smoke. Pulled plugs for clean and regap.

Motor idles better now, but I didn't gain the compression I was hoping for. Approx 65-70 on top and bottom. However, pressure is leaking down slower. So I would guess the rings are sealing better? Just expected better results on compression numbers.

Should I try this again? Or am I just wasting my time?

Sorry for the length. Thanks if you read this far.

 

[iwombat]

Re: Decarb and Compression

If your rings are really coked up, it may take several sessions to free them up thoroughly. Keep going until you don't see any improvement.

 

[BoatBuoy]

Re: Decarb and Compression

You wrote "However, pressure is leaking down slower." Are you doing a conventional compression test, or are you doing a leak-down test?

 

[fishchaser]

Re: Decarb and Compression

You wrote "However, pressure is leaking down slower." Are you doing a conventional compression test, or are you doing a leak-down test?

Well, I'm gonna show my ignorance here. I don't know the difference. Here is what I'm doing.

Removing both plugs. Attaching pressure gauge to top cylinder. Giving the rope several pulls and watching for highest reading on gauge. Repeating procedure for bottom cylinder.

After I'm finished pulling, the pressure on the gauge is going down more slowly than it was prior to the decarb.

Please educate me if I'm messing this up.

 

[darrklim2]

Re: Decarb and Compression

Well, I'm gonna show my ignorance here. I don't know the difference. Here is what I'm doing.

Removing both plugs. Attaching pressure gauge to top cylinder. Giving the rope several pulls and watching for highest reading on gauge. Repeating procedure for bottom cylinder.

After I'm finished pulling, the pressure on the gauge is going down more slowly than it was prior to the decarb.

Please educate me if I'm messing this up.

This would be a compression test.

 

[BoatBuoy]

Re: Decarb and Compression

The compression, as generally used here, is the highest reading attained when cranking the engine. Most compression guages have a valve that won't allow the release of pressure from the guage until a valve release button is pushed.

A leak-down test is performed by applying a prescribed head of air on the cylinder and timing it's pressure fall. I've never seen it done on outboard, just aircraft where there are leak-down specs.

In your case, timing the pressure fall is meaningless because you don't know where the piston is stopping relative to the ports.

 

[fishchaser]

Re: Decarb and Compression

Thanks for the quick responses and good info.

 

[reload]

Re: Decarb and Compression

I would not trust the compression gauge maybe. It sounds like it is only the schradder valve leaking but how do you know what the max compression maybe if it held.

Just my two cents.

 

[jaymasta]

Re: Decarb and Compression

I don't think your are supposed to pull both plugs at the same time, I think you are supposed to take one out hook up your guage to the plug you pulled ground the wire to the block and then do it and vice versa for the other cylinder, Iam pretty sure you can cause damage doing it that way...but not 100% sure

 

[iwombat]

Re: Decarb and Compression

You pull both plugs and ground the wire(s) you're not hooking up to the tester. If you leave the plugs in you can't turn the engine over fast enough to get a good reading.

The spare wires can be grounded with a nail, or screw placed against the block.

 

[Scaaty]

Re: Decarb and Compression

QD's will run OK on 60, (remember..just a touch old now here), but 70 to 90 (that would be a well taken care of old QD on 90) is a better bet..keep on decarbing a few more overnights of Deep Creep through the carb 'till it quits, then soak the cylinders (through the plug holes) in the horizontal position..again, do a 24 hour ordeal on it..
And again, every gauge will read different